The invention may be implemented within electronic devices such as computers, mobile terminals, etc, which are each equipped with a display unit. Images are usually produced by software applications according to a raster direction which is determined by each application itself. Then, the images are to be displayed using a screen raster direction which may be different from the raster direction of the images as produced by the application. For example, the screen may be soldered within an electronic device with fixed screen raster direction, e.g. along image column direction, whereas the raster direction of the images as-produced by the application may be different, e.g. along image line direction. Other situations where the application raster direction and the screen raster direction may be different from each other appear when the electronic device may be changed in orientation, and the image is rotated in accordance with the device orientation for the image to appear always with top upwards. In all cases, the image rotation is required to be as fast as possible, in particular when the image rotation is triggered automatically upon detecting a rotation of the electronic device.
A basic approach is to perform a rotation while recopying the buffer from one memory location to another. It is however very costly in terms of bandwidth. A more optimal and common method is to perform an in-flight rotation: such image raster rotation consists in providing two buffer memory arrays which allow in-flight raster rotation during the transfer of each image from an application output memory to the display unit. Both buffer memory arrays are implemented in parallel in a ping-pong mode between an image-writing sequence and an image-reading sequence. For example, each image is output by the application according to horizontal raster direction, and is to be fed into the display unit according to vertical raster direction. In the whole description hereafter, horizontal direction and vertical direction refer to the image itself based on its image content. A next image or image strip is loaded into one of the buffer memory arrays while a former image or image strip previously loaded into the other buffer memory array is being currently transferred from this latter buffer memory array to the display unit. Both loading and transferring sequences are performed simultaneously and independently from each other, using separately both memory buffer arrays in turn. Both buffer memory arrays are swapped between loading an image or image strip from the application output memory and then transferring this image or image strip to the display unit. The scanning order of each image is the application raster direction for loading the image into one of the buffer memory arrays, whereas the scanning order which is used for transferring each image to the display unit is the screen raster direction. Therefore such in-flight image raster rotation requires two memory buffer arrays. Such pair of memory buffer arrays may be embedded in the circuit silicon for high speed operation. But, the implementation of two memory buffer arrays requires a doubled silicon area within the integrated circuit, which causes cost increase as a consequence.
Document U.S. Pat. No. 7,307,635 discloses another method for performing image raster rotation during the transfer from the application output memory to the display unit. This other method requires only one intermediate buffer memory which is of line memory type. The pixel values of a first image strip are loaded into the line buffer memory according to the image pixel order set by the application raster direction. Then, the pixel values are transferred to the display unit according to the screen raster direction, and once a pixel value of the first image strip has been transferred from a memory cell of the line buffer memory to the display unit, another pixel value pertaining to a next image strip is loaded into the same memory cell. But the successive loading sequences of image strips to the line buffer memory and the transferring sequences from the line buffer memory are carried out using variable offsets for the cell positions within the line buffer memory. These offsets must be calculated again for each new image strip which is handled, and such calculation is quite complicated for being implemented at hardware level.
In addition, successive loadings of pixel values into the buffer memory line do not relate to adjacent memory cells, which does not allow burst-handling of the pixel values. Similar incompatibility with burst-handling also appears for the transfer of the pixel values from the buffer memory line to the display unit. As a consequence, for transferring N pixel values from the application output memory to the display unit, the method of document U.S. Pat. No. 7,307,635 requires N separate accesses to the buffer memory for pixel value writing and also N additional separate accesses for pixel value reading. Because of this, the buffer memory line must be of a dual port memory type, which is more expensive and complicated to implement.
Starting from these existing solutions, one object of the present invention consists in proposing a new method for changing an image raster direction, which requires a single buffer memory.
A further object of the invention is that the buffer memory is limited in size so that it can be silicon-embedded within an integrated circuit without much cost increase.
Still another object of the invention is that the buffer memory can be of single port memory type.
Still another object of the invention consists in proposing a method for changing an image raster direction, which is easy to implement. In particular, the invention method does not involve complex calculation of offset values for selecting the cells of the buffer memory in which writing and reading operations are to be executed.
Then, a further object of the invention is that a controller used for controlling the execution of the invention method can be also silicon-embedded. Put another way, the controller can be hardware-implemented.
Still another object of the invention is that the invention method is compatible with burst-handling of the image pixel values.